The present invention relates to reduction of iron bearing materials such as iron ore to metallic iron nodules (known as “NRI”).
Metallic iron has been produced by reducing iron oxide such as iron ores, iron pellets, and other iron sources. Various such methods have been proposed so far for directly producing metallic iron from iron ores or iron oxide pellets by using reducing agents such as coal or other carbonaceous material. Such fusion reduction processes generally involve the following processing steps: feed preparation, drying, preheating, reduction, fusion/melting, cooling, product discharge, and metallic iron/slag product separation. These processes result in direct reduction of iron bearing material to metallic iron nodules (NRI) and slag. Metallic iron nodules produced by these direct reduction processes are characterized by near total reduction, approaching 100% metal (e.g., about 96% or more metallic Fe). Percents (%) herein are percents by weight unless otherwise stated.
Unlike conventional direct reduced iron (DRI) product, the metallic iron nodule (NRI) product has little or no gangue and little or no porosity. NRI is essentially metallic iron product desirable for many applications, such as use in place of scrap in steelmaking by electric arc furnaces. Metallic iron nodules are generally as easy to handle as taconite pellets and DRI, and are a more efficient and effective substitute for scrap in steel making by electric arc furnace (EAF) without extending heat times and increasing energy cost in making steel.
Various types of hearth furnaces have been described and used for direct reduction of NRI. One type of hearth furnace used to make NRI is a rotary hearth furnace (RHF). The rotary hearth furnace is partitioned annularly into temperature zones between a supply location and the discharge location of the furnace. An annular hearth is supported rotationally in the furnace to move from zone to zone carrying reducible material the successive zones to reduce and fuse the reducible material into metallic iron nodules, using one or more heating sources (e.g., natural gas burners). The reduced and fused NRI product, after completion of the process, is cooled to prevent reoxidation and facilitate discharge from the furnace. Another type of furnace used for making NRI is the linear hearth furnace such as described in U.S. Pat. No. 7,413,592, where similarly prepared mixtures of reducible material are moved on moving hearth sections or cars through a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the charging end and discharging end of a linear furnace while being heated above the melting point of iron. As one example, a method for use in production of metallic iron nodules is disclosed in U.S. Pat. No. 7,628,839.
It has been desired in the production of NRI to reduce the amount of time for reduction and fusion of reducible material in forming metallic iron nodules while reducing the amount of sulfur in the nodules and limiting the formation of micro metallic iron nodules. Micro metallic iron nodules (called micro-nodules or micro NRI) include small particles of agglomerated iron having a size between about 20 mesh and about 3 mesh.
What is disclosed is a method for use in production of metallic iron nodules comprising the steps of
providing a hearth comprising refractory material;
providing reducible mixture above at least a portion of the refractory material, the reducible mixture comprising at least reducing material and reducible iron bearing material;
forming the reducible mixture to comprise:                a quantity of reducible iron bearing material,        
forming the reducible mixture to comprise:                a quantity of reducible iron bearing material,        a quantity of first carbonaceous reducing material of a size less than about 48 mesh of an amount between about 65 percent and about 95 percent of a stoichiometric amount necessary for complete iron reduction of the reducible iron bearing material, and        a quantity of second carbonaceous reducing material with an average particle size greater than average particle size of the first carbonaceous reducing material and a size between about 3 mesh and about 48 mesh of an amount between about 20 percent and about 65 percent of a stoichiometric amount of necessary for complete iron reduction of the reducible iron bearing material;        where amount of first carbonaceous reducing material and second carbonaceous reducing material provide total reducing material carbon between about 110 and 150 percent of a stoichiometric amount necessary for complete iron reduction of the reducible iron bearing material, and        thermally treating the reducible mixture in the presence of other carbonaceous material separate from the reducible mixture to form one or more metallic iron nodules by melting.        
The quantity of first carbonaceous reducing material may be of an amount between about 80 percent and about 90 percent of a stoichiometric amount necessary for complete iron reduction of the reducible iron bearing material. Alternatively, the quantity of first carbonaceous reducing material may be of an amount between about 85 percent and about 95 percent of a stoichiometric amount necessary for complete iron reduction of the reducible iron bearing material. In yet another alternative, the quantity of first carbonaceous reducing material may be of an amount between about 65 percent and about 75 percent of a stoichiometric amount necessary for complete iron reduction of the reducible iron bearing material. The quantity of second carbonaceous reducing material being of an amount between about 20 percent and about 50 percent of a stoichiometric amount necessary for complete iron reduction of the reducible iron bearing material.
The basicity B2 of the reducible mixture may be between 1.5 and 2.3. Alternatively, the basicity B2 of the reducible mixture is between 1.9 and 2.3.
The first carbonaceous reducing material may be of a size less than about 65 mesh. Alternatively, the first carbonaceous reducing material may be between about 65 mesh and about 100 mesh.
The second carbonaceous reducing material may be of a size between about 6 mesh and about 65 mesh. Alternatively, the second carbonaceous reducing material may be of a size between about 6 mesh and about 48 mesh.
The first carbonaceous reducing material may include at least two sources of carbonaceous material, at least one source being fines less than about 48 mesh from a source of carbonaceous material in the second carbonaceous reducing material.
The first carbonaceous reducing material may be a carbonaceous material with between 2 and 40% average volatiles, and the second carbonaceous reducing material may be a non-caking carbonaceous material with less than 10% average volatiles. Alternatively, the second reducing material may be a non-caking carbonaceous material with between 1 and 8% volatiles.
The reducible mixture may be formed into agglomerates. In one alternative, the second carbonaceous reducing material is of a size less than 20 mesh and the reducible mixture is formed into balls.